The decrease in the availability of resources drives ever-increasing efforts in the extraction of raw materials and energy. As a result, offshore drilling for oil and gas has to be carried out in increasingly greater depths. For the safe operation of such deep-water drilling carried out from drilling rigs or drilling vessels, extensive security systems are provided on the seabed, which systems are functionally associated with the transition region between the borehole and drill pipe or production tubing. A blowout preventer (BOP) constitutes an important piece of equipment that is a standard safety device in such deep-water drills. This device instigates a quick closing of the wellhead, drill pipe and/or the conveying tube in the event of danger.
To ensure the reliable function of a blowout preventer, a pressure medium for the hydraulic actuation having a correspondingly high working pressure has to be provided. Since transporting a sufficient amount of a hydraulic fluid having a sufficiently high operating pressure from a drilling platform or a drilling vessel at the surface of the water to the seabed below is extremely difficult, the state of the art, cf. U.S. Pat. No. 6,418,970 B1, uses the hydraulic operating pressure required for the actuation of corresponding deep-water installations in these devices at the location of the deep-water installation itself. To this end, the surrounding pressure of the deep sea, i.e., the high pressure of the deep water, is used to produce the required hydraulic operating pressure. The ambient pressure of the deep sea is applied to a piston in a cylinder. The resulting piston movement is used to transfer the pressure to the pressure fluid.
Despite the advantages resulting from the production or transfer of the operating pressure at the place of action, the operating characteristics of the known devices are not satisfactory. The use of seawater for the operation of the cylinder assembly is questionable in several respects. On the one hand, there is a risk of pollution caused by the entry of sediment particles and the like, or by microorganisms introduced in conjunction with the seawater. On the other hand, there are drawbacks due to the extremely corrosive seawater. To counter the corrosion, the prior art needs to provide the cylinder arrangement with suitable linings and/or to manufacture it of suitably corrosion-resistant materials to reduce corrosion and/or to reduce the coefficient of friction, increased by accumulations, for piston movements. Despite these measures, difficulties are caused by saltwater deposits, for instance by potassium stearate.
To overcome these challenges, a device for transmitting a hydraulic working pressure in a hydraulic fluid to the hydraulic pressure actuating devices of deep-water installations, especially deep-water drilling, is suggested in DE 10 2011 009 276 A1. In a cylinder arrangement, a first pressure chamber for the hydraulic fluid, a movable piston assembly for changing the volume of this pressure chamber and at least a second pressure chamber are provided. The ambient pressure of the deep sea can be applied to the second pressure chamber for a movement of the piston assembly generating the operating pressure in the first pressure chamber. Also, a pressure accumulator associated with the cylinder assembly is provided in the form of a bladder accumulator, whose the movable separating member separates a chamber connected to the sea water from a chamber for movement. The actuating chamber contains an actuating fluid and is connected to the second pressure chamber to pressurize it to the deep-sea pressure by the actuating fluid.